Hand-held devices including wireless communication devices such as mobile phones, tablets, phablets, and personal digital assistant (PDA) are good candidates for using wireless power transfer (WPT) technology for battery charging. In WPT, energy is transferred from a power source to an electrical load without using an electrically conductive medium such as a wire for power transmission. Instead, the power transfer can take place wirelessly by using time-varying electric, magnetic, or electromagnetic fields. The wireless energy transfer is from a wireless power transmitter connected to power source to one or more wireless power receivers that receive the energy through an intervening space.
Common WPT technologies include magnetic resonance coupling, electromagnetic induction, and radiative power transfer. The magnetic resonance and electromagnetic induction solutions are used for short distances, whereas the radiative solution, also known as power beaming, is a far-field solution. The power beaming technique is mostly considered for use by solar power satellites and drones that receive electrical power from one or more beams of electromagnetic radiation such as microwave or laser. In the magnetic resonance coupling, the energy transfer is based on resonance between magnetic coils of the power transmitter and the power receiver devices. In the electromagnetic induction technique, inductive coupling between coils of the power transmitter and the power receiver devices is used for the wireless power transfer. Both of the magnetic resonance coupling and electromagnetic induction techniques are based on high-frequency electromagnetic radiation in the range of a few hundred KHz to a few MHz (e.g., about 100 KHz-10 MHz), and their transmission efficiency is a function of the product of the coil Q-factor and the coupling coefficient between the coils.